The fruits of Piper retrofractum Vahl. contain a variety of compounds, but no phytotoxic compound with plant growth inhibitory activity has been reported. Therefore, the phytotoxic activity of P. retrofractum fruit extracts was investigated and found the reduction of seedling growth of cress, lettuce, alfalfa, barnyard grass, Italian ryegrass, and jungle rice. Seven bioactive compounds were isolated from the fruit extracts and identified as 3-phenylpropanoic acid (1), (2E,4E)-methyl piperate (2), (2E,4Z)-methyl piperate (3), piperlonguminine (4), dihydropiperine (5), isochavicine (6), and piperine (7). There have been no reports for their phytotoxic activity. The activity of these compounds was determined against the germination and seedling growth of cress and barnyard grass. The total germination percentage (GP) of cress was significantly inhibited by 1, 2, and 6 (40–100% inhibition), while its germination index or progressive total of cumulative germination (GI) was the most sensitive to 1, leading to the longest time required for 50% germination (T50) at 57 h behind control. However, the seedling growth of cress was highly inhibited by 6 with I50 values of 11 µM. The compounds 1 and 5–7 showed significant inhibition on GP of barnyard grass by 55–47%, while 5 and 1 effectively reduced GI and extended T50 to 42–48 h behind control, and 7 showed the most growth inhibition with I50 values of 37 µM. The inhibitory effects of the seven compounds indicate the phytotoxic property of P. retrofractum fruits, which 1, 5–7 may be the main compounds responsible for the phytotoxicity.
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